A digital color image forming apparatus, such as a full color copying machine and a full color laser printer, uses four color toners, i.e., Y (yellow), M (magenta), C (cyan), and K (black), and develops and superposes images of those color toners to reproduce a color image. To read an image on an original placed on a platen and to make a color copy, the original image is optically read and the read out signals are converted into developing signals. Thus, four separate copying processes, i.e., one for each color, are required to make a full color copy.
Generally, originals are categorized into character originals, photograph originals, print originals, and originals in which character, photograph, and print coexist (referred to as "integrated" originals). In reproducing a half tone image, such as a photograph, a smoothing process is performed to remove noise and mesh-dot components of the image signal and to improve definition and tone reproduction. Consequently, a smooth halftone image will be provided.
If the process applied to reproduce a halftone image is applied to reproduce a binary image, such as a character image, the edge portions of the binary image will be blurred. That is, in reproducing the binary image, the edge portions must be emphasized to increase the sharpness of the image. Therefore, to reproduce both photographic and character images with satisfactory levels of picture quality, it is necessary to compromise between the smoothing process, such as removal of the noise and mesh-dot component, and the edge emphasis process. In other words, edge emphasis processing must be performed to an extent that the image is not blurred, while at the same time the image signal must be improved to facilitate the reproduction of halftone images, such as photographs.
If the smoothing process and the edge emphasis process are applied to a character original, photograph original, print original, and integrated original, the resultant image will not be too bad in image quality as a whole. However, when individual images are examined the reproduced image of the photograph original looks rough or unsmooth, because the edge portions are somewhat emphasized. Some blur will be noticeable in the reproduced image of the character original. Clearly, it is difficult to reproduce halftone images and binary images such that both have excellent image quality.
While optimal smoothing processes and edge emphasis processes have been known for halftone images and binary images, the mere combination of these processes has not resulted in reproducing integrated images with satisfactory image quality. It has proven to be difficult to select parameters so that both halftone and binary images have satisfactory image qualities.
It is common for copying machines to provide a reduction/ enlargement function. The parameters for the smoothing and edge emphasis processes are selected for 100% magnification. Accordingly, when a copy is made in a reduction mode or an enlargement mode, the picture quality of the reproduced image is deteriorated. To be more specific, in the case that an image is enlarged or reduced and the parameters for the smoothing process and the edge emphasis process as optimally set at 100% magnification are used, unnatural image emphasis will be observed in the reproduced image. An enlarged image will suffer from detail blur, and any detail blur noticeable at 100% magnification will be even more noticeable when the image is enlarged. When the image is reduced, the removal of Moire is imperfect and appears in the reproduced image.